1. Field of the Invention
This invention relates to regenerative braking and energy storage systems for motor vehicles.
2. Description of the Related Art
Electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) provide highly desirable benefits in being able to reduce consumption of liquid transportation fuels. However, industry has not yet found a battery technology that is sufficiently low cost and durable to achieve widespread commercialization and consumer acceptance of these vehicles without large subsidies.
The predominate Nickel Metal Hydride (NiMH) and Lithium Ion (Li-Ion) battery technologies cost $500 to $1000 or more per kilowatt-hour of energy storage capability. As a result, for a hypothetical EV or PHEV sedan with a 16 kilowatt-hour lithium ion battery pack for a 40-mile or more electric-powered range, the battery pack alone could add $16,000 to the cost of the vehicle, thereby significantly limiting the potential market for the vehicle. For a larger, commercial vehicle, the battery pack can cost two to five times more, affecting the potential market for larger vehicles as well. In addition, NiMH and Li-Ion batteries face potential resource constraints for the metals necessary for the batteries in the event of widespread deployment, which may further increase costs or reduce availability. Cooling costs and concerns to avoid overheating of the batteries is also a challenge. Significant technology breakthroughs are needed for NiMH and Li-Ion batteries to meet the demand for a commercially viable EV or PHEV at a reasonable cost.
In contrast, lower cost traditional lead-acid batteries cost closer to $100 per kilowatt-hour of energy storage capability. However, traditional lead-acid batteries have a much lower cycle life (e.g., less than 1000 cycles unless the depth of discharge is significantly limited to extend life). As a result, an EV or PHEV with a lead-acid battery pack would be expected to need battery replacement after just 2-3 years unless the all-electric range is dramatically limited to maintain battery life. In addition, traditional lead acid batteries use a large quantity of lead, which must be managed to reduce harmful environmental effects.
Lead acid batteries are also larger and heavier than NiMH and Li-Ion batteries for the same amount of energy storage, thereby presenting packaging challenges and escalating weight-related costs as the battery pack increases in size and weight (e.g, at some point, the battery weight necessitates greater structural support for the vehicle, which means even greater vehicle weight, all of which increases cost and the amount of energy needed to move the vehicle, therefore offsetting fuel efficiency gains for an HEV or PHEV; and fitting the battery pack within a vehicle without significantly reducing cargo or passenger space becomes difficult). Industry has therefore turned away from lead acid batteries as a technology for future EVs and PHEVs, particularly PHEVs with a significant engine-off range (e.g., a range of about 20 miles or more, and battery packs of around 12 or more kilowatt-hours).
As alternatives, two small companies, Firefly Energy and Axion Power, have developed advanced lead-acid batteries with a greater cycle life (e.g., about 2000 cycles at a 60% depth of discharge for Firefly, and about 1600 deep-discharge cycles for Axion Power), and with a lighter weight (and much less lead content for Firefly) than traditional lead-acid batteries, albeit at a cost premium above other lead acid batteries (but still potentially less than one-half the cost of Li-Ion batteries). However, the cycle life for these advanced lead acid batteries remains insufficient for the expected life of a motor vehicle. For purposes of this application, the term “lead acid batteries” as used hereafter encompasses traditional lead acid batteries and advanced lead acid battery technologies such as Firefly's and Axion Power's, unless otherwise specified.
Therefore, there remains a desperate need in the art for a cost-effective and durable energy storage system for EVs and PHEVs.
Furthermore, while many electric storage batteries can have “optimal” charging efficiencies (defined herein as a battery charging efficiency of 80% or more) during regenerative braking for very light braking events (and high discharge efficiencies for very light electric-powered acceleration events), it has been found that average charging efficiencies in normal vehicle braking are much lower, due in part to charging the battery at higher than optimal charging rates in order to help meet driver braking power demands. For example, a sample battery may have an 85% “roundtrip” efficiency (taking into account both the charging efficiency and the discharging efficiency) at a small charge/discharge current X, an 80% roundtrip efficiency at a charge/discharge current 2×, a 70% roundtrip efficiency at a charge/discharge current 4×, 60% roundtrip efficiency at current 8×, 50% roundtrip efficiency at current 12×, and 40% roundtrip efficiency at a charge/discharge current 16×. See, e.g., Yufang Li, et. al, “An Analysis of Electric Assist Control Strategy for Hybrid Electric Vehicles and Simulation,” Journal of Asian Electric Vehicles, Vol. 2, No. 1, pp. 527-529, FIG. 5 (2004).
Therefore, there remains a desperate need in the art for a more cost-effective and efficient way to capture and store braking energy in an EV, HEV, or PHEV.